The present invention pertains to a pressure sensor for measuring absolute dynamic pressure, and more particularly, a sensor comprising a frame, a diaphragm arranged in the frame and attached thereto along parts of the outer edge of the diaphragm, where the diaphragm has a measurement side toward the surroundings and a rear side, and further a reference chamber behind the diaphragm rear side and a restriction connecting the reference chamber to the surroundings, as well as a signal-providing element arranged to detect mechanical stress in an attachment part in the outer diaphragm edge. The invention also pertains to a photoacoustical gas detection sensor, a microphone and a hydrophone based upon use of the pressure sensor. Further, the invention pertains to a method for manufacturing a pressure sensor for dynamic absolute pressure, and a method for manufacturing a photoacoustical gas detection sensor.
Pressure sensors based on movement/flexure of a diaphragm are as a starting point only able to measure differential pressures. E.g. in silicon pressure sensors the sensor element consists of a diaphragm which on respective sides thereof is in contact with a fluid under pressure, and which diaphragm flexes dependent on the pressure difference between the fluids.
In order to manufacture an absolute pressure sensor with a differential sensor as a starting point, one has to provide a substantially closed chamber with a reference fluid on one side of the diaphragm. The volume of this chamber is called the reference volume. The reference volume, and consequently the reference pressure, will vary with temperature. Hence, temperature changes will introduce errors in the measurement system.
Another aspect that must be taken into consideration in connection with the reference volume, is that when the diaphragm flexes toward the reference chamber, the fluid in the reference volume will be compressed, and thereby form a pressure, depending on the compressibility of the fluid. This phenomenon is termed pressure feedback, and such pressure feedback affects the sensitivity of the system. In order to minimize the pressure feedback, the reference volume must be large, so that the diaphragm flexing volume constitutes only a small fraction of the reference volume. (Alternatively, the diaphragm must be rigid, and this means at the same time low sensitivity.)
One way of overcoming the two above mentioned problems of temperature errors and pressure feedback, is to arrange a reference chamber with a vacuum behind the diaphragm. A temperature increase will then not give any increase of pressure in the reference chamber, and pressure feedback cannot arise. However, such an embodiment with a vacuum in the reference volume, will lead to limitations in the pressure values that can be measured. If the static pressure is approximately 1 bar, as will normally be the case in the surroundings, the diaphragm must be dimensioned to withstand a pressure difference of 1 bar if a vacuum is used in the reference volume, and such a diaphragm will be rather poorly suited for measuring very small pressures, in particular dynamic pressures e.g. in connection with acoustical oscillations. When these small, dynamic pressures are to be measured, the diaphragms must be dimensioned in relation thereto, and a static pressure difference of e.g. 1 bar will then possibly lead to destruction of such a diaphragm. In other words, in connection with measuring dynamic pressures having small pressure values, e.g. in the 1 pascal range, one must use a reference volume that contains a fluid.
The sensitivity of the measurement system will also depend on the volume displacement of the diaphragm. The volume displacement is flexure volume per pressure unit, i.e. the volume occupied by the flexed diaphragm in the reference chamber, divided by the pressure. In general, a good sensitivity implies a large flexure/volume displacement. This can easily be appreciated by considering a very thin diaphragm that flexes easily when a pressure differential is present. The disadvantage of a large volume displacement is that the pressure feedback increases, and the sensitivity of the sensor is reduced.
If it is desirable to measure rapidly changing, dynamic pressures, typically in connection with sound oscillations, it is possible to make a small channel into the reference chamber, thus letting the surrounding medium into the chamber. At the outset this will make the diaphragm more robust to exposure to atmospheric pressure, temperature changes, or to handling. Such a channel or opening that connects the reference volume to the sensor surroundings, or more generally to the pressure input of the sensor, is called a restriction. This is because the opening/channel is so narrow that it will take a long time until the pressure inside the reference volume is equalized in relation to the external pressure. The restriction has the effect of a negative feedback regarding slow changes, i.e. low frequencies in the pressure oscillations. The cross section area and the length of the restriction represent a flow resistance, equivalent to an electrical resistance, and the reference volume multiplied by the fluid compressibility, represents a reservoir, equivalent to an electrical capacity, and together the restriction and the reference chamber then operate as a first order lowpass filter for the pressure oscillations, since the low frequencies have sufficient time to get through the restriction and thereby influence both sides of the diaphragm, while the high frequencies will merely affect the diaphragm side facing the surroundings/the pressure input, and therefore will be measurable. In other words, the sensor will be sensitive to frequencies higher than the corner frequency of this filter. By shaping the restriction and the reference chamber in a suitable manner, it is possible to control the corner frequency of the filter, and in this way the measuring range of the sensor. If it is desirable to achieve an extended measuring range down toward low frequencies, then a large reference volume will be advantageous, since this provides a low corner frequency.
However, size is often an important criterion in manufacturing a sensor that fulfils requirements set by an application. In order to make a small sensor, it is of course important that the reference volume is made as small as possible, but with a small reference volume, and when it is desirable to measure relatively low frequencies, one must prepare a very narrow restriction, and the diaphragm must give an extremely small volume displacement. Also if it is desirable to manufacture pressure sensors in planar technology, i.e. in batches, the size of the reference volume should be restricted. The advantage of this type of manufacturing is that the sensors can then be manufactured at a reasonable cost.
The most common application regarding measurement of dynamic pressures, is in sound measurement, which comprises dynamic pressures all the way down into the xcexcPa range. When it is desirable to make measurements in these pressure ranges, static pressure variations are quite destructive. Variations due to high and low pressures may amount to several tens of kilopascals (1 kPa=10 mBar). If one wishes to measure dynamic pressures under water at various depths, the static pressure changes may be even much larger. When dynamic pressures shall be measured, one will often use sensor elements that are sensitive only to dynamic pressure, and the most common example of such a sensor element is a piezoelectric crystal. Such crystals have many good characteristics, inter alia a low price, a high natural frequency and a low sensitivity to acceleration. However, it is a disadvantage that such piezoelectric crystals have a limited stability over time, as well as poor low frequency characteristics (in comparison with monocrystalline piezoresistive structures).
Therefore, one has lately to an increasing degree changed to using diaphragm sensors made by silicon, which material exhibits better stability. Embodiments of such diaphragm sensors have been mentioned above. One has tried to manufacture diaphragm sensors with restrictions in order to provide a high sensitivity, and by means of cost reasonable manufacturing technologies. For instance Norwegian patent application no. 97.1201 discloses a pressure sensor based on solid state technology, where e.g. a semiconductor chip, preferably with silicon as a start material, is processed to comprise a relatively thick frame, an intermediate, thin diaphragm and a central, thick block that can be pushed down by an overpressure, a reference chamber being provided underneath the block and diaphragm, between the semiconductor chip and a substrate thereunder, e.g. a glass substrate. At least two rigid and thick beam connections between the central block and the surrounding frame provide areas where high mechanical stresses are induced when the block exhibits a deflection due to a pressure variation, and signal-providing piezoresistive elements are located in these areas, which in their turn are located where the beams pass on to the frame and the block respectively. Pressure sensors of this type can be batch manufactured from a larger semiconductor wafer that is bonded to a larger substrate disc, then to be cut into single sensors in the end. This previously known pressure sensor can also be equipped with a restriction to provide a connection between the reference chamber and the sensor pressure input, so that a suitable corner frequency can be provided for the low end of the frequency measuring range. However, this known sensor has disadvantages like a large area and a significant volume displacement due to the thin diaphragm areas, and these features do not contribute to the sensitivity. Besides, the manufacturing process is relatively complicated and costly, and a sensor with a closed reference chamber is not very suitable for batch production.
Norwegian patent no. 300,078 discloses a photoacoustical gas detector having a chamber that contains a gas type to be detected somewhere else. The chamber has been manufactured by bonding together two silicon or quartz plate elements prepared by the use of planar technology. The chamber has windows for transmission of pulsed IR radiation, and a pressure sensor with a diaphragm is arranged above a closed space that communicates with the chamber. However, this gas detector exhibits clear limitations regarding practical implementation and manufacturing costs. The location of a signal-providing element in relation to the diaphragm, in order to achieve high sensitivity, is not mentioned in the patent. Nor are any restrictions between a measurement chamber and a reference chamber mentioned in that publication.
U.S. Pat. No. 5,633,552 discloses a pressure sensor which comprises a chamber and a slab attached to a frame lying on top of the chamber. The slab is attached to the frame along one of the sides, and constitutes sort of a cantiliever beam from the frame. The remaining three sides of the slab are separated from the frame by a vertical slit. A piezoelectric element is placed on the slab in the area where it is attached to the frame. The slit width is approximately 10 xcexcm. This device has a high sensitivity (2 mV/xcexcBar for frequencies situated in the range 100-1000 Hz), the slab being easily movable by means of small pressure differences. In order to maintain the slab flat in the resting position, it is manufactured in the form of three sandwiched layers having different internal stresses. The device has a relatively large volume displacement, and therefore large pressure feedback. In addition thereto, the use of a piezoelectric element for measuring mechanical stress will lead to unstable measurements, since a piezoelectric element is not as stable over time and with regard to temperature, as e.g. a piezoresistive element. The sensor indicated in the publication must be provided with a relatively large reference chamber in order to compensate for the effect of the large volume displacement. This makes batch manufacturing of the complete sensor very difficult. The top part of the sensor, i.e. the sides and roof (slab) of the reference chamber are batch manufactured as parts of a larger wafer, but thereafter the wafer, including chamber sides and slabs, is cut into a plurality of top parts, and each respective top part is laminated to a bottom. This manufacturing method is costly and not very efficient.
The purpose of the present invention is in a first aspect to provide a pressure sensor for measuring absolute, dynamic pressure, which sensor satisfies high sensitivity requirements, and which sensor is provided with a small reference chamber, and furthermore, it can be manufactured in planar technology to provide a reasonable manufacturing cost. Other aspects of the invention will appear below.
In accordance with the present invention, the purpose is achieved by a pressure sensor of the type indicated in the introduction, and which is characterized in that the diaphragm is attached to the frame along at least two parts of the outer diaphragm edge, at least one of the diaphragm attachment parts comprising an area where mechanical stresses caused by the pressure, are concentrated, the signal-providing element being arranged in this area, and that remaining parts of the outer edge of the diaphragm are separated from the frame by slits constituting the restriction.
In an important embodiment, the diaphragm and the frame are in one piece formed from a planar material disk, the slits then being slits all the way through the disk thickness, said area then being a transition area from the diaphragm to the frame.
The measurement side of the diaphragm is preferably in the same plane as the adjacent surface of the frame.
In a preferred embodiment, the material disk is of silicon, and the signal-providing element is then preferably a monocrystalline piezoresistive element.
In a further preferred embodiment, the slits that as a starting point have a depth dimension substantially transverse to the measuring side of the diaphragm, continue in under the rear side of the diaphragm, the frame or the wall of the reference chamber being shaped with a shoulder closely adjacent the rear side of the diaphragm in an area along the diaphragm edge.
Preferably, the dimensions of the restriction are adapted to provide a flow resistance that together with the volume of the reference chamber constitutes a low pass filter for the pressure equalising rate of the sensor, said filter having a corner frequency adapted to the frequency range of the dynamic pressure variations to be measured.
Several diaphragm attachment parts may be formed, arranged with intervals along the outer edge of the diaphragm, and these attachment parts will comprise stress concentration areas, as mentioned above.
Preferably, the sensor comprises at least one piezoresistive element incorporated into said area or said areas, as a signal-providing element, preferably as an element in bulk material or as a thin film element on the surface.
In another aspect of the invention, there is provided a photoacoustical gas detection sensor for detection and/or concentration determination of a particular gas in the surroundings of the sensor, wherein the sensor comprises a measurement chamber filled with the gas in question, a radiation source spaced from the measurement chamber, and inside or adjacent to the measurement chamber a pressure sensor for measuring absolute dynamic pressure in the measurement chamber. The measurement chamber is equipped with at least one wall area or window that is transparent to radiation from the source with a wavelength that can be absorbed or scattered by the gas to be detected. The pressure sensor comprises a frame, a diaphragm arranged in the frame and attached thereto by the outer diaphragm edge, said diaphragm having a measurement side toward the measurement chamber and a rear side. Further, the pressure sensor comprises a reference chamber that substantially is not exposed to radiation, behind the rear side of the diaphragm, and a restriction connecting the reference chamber to the measurement chamber, and a signal-providing element arranged to detect mechanical stress in an attachment part at the outer diaphragm edge. The gas detection sensor is characterized in that the diaphragm is attached to the frame along at least two parts of the outer diaphragm edge, at least one of the attachment parts of the diaphragm comprising an area in which mechanical stresses caused by the pressure, are concentrated, and that remaining parts of the outer edge of the diaphragm are separated from the frame by slits constituting the restriction, the reference chamber and the measurement chamber constituting together a closed system filled by the same gas.
The measurement chamber and the reference chamber of the gas detection sensor are manufactured by laminating and etching techniques and with substantially similar dimensions on respective sides of the frame with the diaphragm and the slits, in order to constitute a compact unit.
In a preferable embodiment of the gas detection sensor in accordance with the invention, the diaphragm has a light reflecting coating on one of its sides, a wall area of the measurement chamber opposite to the diaphragm then having a light transparent area/window.
In a further aspect of the invention, there is provided a microphone comprising a pressure sensor for measuring absolute dynamic pressure, a microphone housing and signal wires, and the microphone is characterized in that the pressure sensor is of the type indicated in the first aspect of the invention.
In a further aspect of the invention, there is provided a hydrophone comprising a pressure sensor for measuring absolute dynamic pressure, a hydrophone housing and signal wires, and the hydrophone in accordance with the invention is characterized in that the pressure sensor is of the type indicated in the above first aspect of the invention.
A further aspect of the invention comprises a method for manufacturing a pressure sensor for dynamic absolute pressure, of the type indicated in the first aspect of the invention, and wherein the diaphragm and the frame are in one piece formed from a plane material disk, the slits then being slits all the way through the disk thickness, so that the mentioned area is then a transition area from the diaphragm to the frame. The method in accordance with the invention is characterized in that in a first, thin disk, preferably of silicon, a restriction is provided in the form of at least two elongate, narrow slits, preferably by means of ionic etching, said restriction defining a diaphragm in relation to a surrounding frame in that the restriction substantially surrounds the outer edge of the diaphragm, except along transition areas from the diaphragm to the frame,
that in a second material disk, preferably of silicon, a recess is etched out, e.g. by wet etching, and
that the two disks are laminated to each other so that the recess constitutes a reference chamber behind the diaphragm, and so that the restriction leads into the reference chamber.
In a first embodiment of the method, the second material disk, prior to laminating to the first disk, is exposed to a further etching step with precise forming of a step along the upper edge of the recess, in such a position that when the laminating is made, the restriction is provided with a bent extension in behind the diaphragm.
In a second embodiment of the method, a third material layer is provided between the two disks by depositing a thin film on to one of the disks, by growing e.g. an oxide on one of the disks or by laminating a third material disk to one of the disks, with an opening adapted to and not smaller than the opening in the frame, and thereafter the remaining one of the two disks is laminated to the disk having the third material layer, so that the restriction is provided with a bent extension in behind the diaphragm by means of the third material layer and the edge of the second disk around the recess.
In a third embodiment of the method in accordance with the invention, the first material disk, prior to laminating to the second disk, is exposed to a further etching step on its rear side, at least in an area around the rear side opening of the restriction, in order to provide the restriction with a bent extension in behind the diaphragm when the laminating is made.
In a favorable embodiment of the method, a number of sensors are batch manufactured by preparing a number of first disks simultaneously and next to each other on a first, larger material disk, a number of second disks are prepared in a corresponding manner next to each other on a second, larger material disk, the two larger material disks are laminated to each other, and single sensors are then provided by sectioning the bonded, larger disks.
In a further aspect of the invention there is provided a method for producing a photoacoustical gas detection sensor of the type indicated according to the second aspect of the invention, and this method is characterized in
that in a first disk, preferably of silicon, a restriction is provided having the shape of at least two elongate, narrow slits, preferably by means of ionic etching, said restriction defining a diaphragm in relation to a surrounding frame in that the restriction substantially surrounds the outer edge of the diaphragm, except along transition areas from the diaphragm to the frame,
that in a second disk, preferably of silicon, a first recess is provided, e.g. by machining or wet etching,
that the two disks are laminated to each other in order that the first recess shall constitute a measurement chamber in front of the diaphragm, and in such a manner that the restriction leads into the measurement chamber,
that in a third disk, preferably of glass, there is provided a second recess, e.g. by machining or wet etching, and
that the third disk is laminated to the first disk in an atmosphere of a predetermined gas in such a manner that the second recess constitutes a reference chamber behind the diaphragm, whereby the restriction also leads into the reference chamber and thereby constitutes a channel between the measurement chamber and the reference chamber, which chambers will both contain this gas after the lamination.
In a favorable embodiment of the method for producing a photoacoustical gas detection sensor, the second disk is provided with a through hole, e.g. by machining, and a fourth disk that is transparent to light which excites the gas in the measurement chamber, is laminated to the second disk to cover the hole.
In a further favorable embodiment, a number of gas detection sensors are batch manufactured by preparing groups of the first, second, third and possibly fourth disks respectively as parts of larger first, second, third and possibly fourth material disks respectively, the respective larger material disks are laminated to each other, possibly in an atmosphere of a certain gas that is to be detected, and single gas detection sensors are then provided by dividing the bonded larger disks.